(Reuters Health) – A small pilot study finds that vaping CBD products might lead to a positive urine test for marijuana, researchers report.

The study tested people after use of cannabidiol (CBD) samples, some containing tiny amounts of THC (the psychoactive ingredient in marijuana). One-third of those who vaped CBD-dominant cannabis had positive urine tests for THC, according to the report in the Journal of Analytical Toxicology.

The researchers hope the findings will alert users of legal CBD products to problems that could arise in drug testing, especially if they don’t know how much THC is in the products. “It’s a common perception that CBD is THC-free,” said senior study author Ryan Vandrey, an associate professor at the Johns Hopkins School of Medicine in Baltimore, Maryland. “But that’s not necessarily the case.”

The study should serve as a warning, Vandrey said, noting that “if the person is subject to workplace or roadside testing or testing for any criminal justice purpose, they could come up positive.”

Vandrey’s team recruited three men and three women willing to ingest 100% pure CBD or cannabis, whose dominant active ingredient is CBD. The cannabis samples contained 10.5% CBD and 0.39% THC. Dummy versions of both forms of CBD were also given to participants.

The study was run on each participant in several iterations, each time with an active and a placebo version of the two products. For example, in one session a participant might be given a placebo CBD capsule together with a vaped high-CBD/low-THC cannabis sample; in another, they might have a 100-mg pure CBD capsule and vape a placebo cannabis containing virtually no CBD or THC.

The 0.39% THC level in the real CBD-dominant cannabis sample is slightly higher than the 0.30% allowed in hemp-derived CBD products, which are legal per new federal government rules. But other products, such as ones labeled high-CBD/low-THC, could have levels comparable to what the study tested, Vandrey and others said.

The only iteration of the experiment that triggered THC-positive tests was the vaped high-low product. Two volunteers tested positive for pot after a single episode of vaping.

While Vandrey allows that the amount of THC in the study’s samples is 0.09% higher than what is allowable in hemp-derived CBD, he points out that people who use more than one dose per day of the hemp product might end up with high enough levels of THC to trigger a positive test because THC accumulates in the body.

And considering that “the packaging of those products most commonly suggests using them two times a day,” that’s not an unrealistic scenario, Vandrey said.

The new study “confirms that even if you are exposed to the low levels of THC in commonly produced cannabidiol products, there is a risk of a positive result in employer drug screens,” said Ziva Cooper, director of research for the Cannabis Research Initiative at the University of California, Los Angeles. “There have been several cases highlighted in the media in which people using CBD products tested positive for THC. So there can be significant legal ramifications.”

Cooper agreed that multiple uses could bump up THC levels in the body. “You would expect it to accumulate,” she said. “So the risk would presumably increase with daily use of those products. It’s really impressive that with only one drug administration two out of six showed positive. Most people wouldn’t think that would happen.”

It highlights the importance of knowing what is in the products you’re using, she added, “and recognizing that a very small amount of THC can push these tests over the edge.”

The study should alert people to the possibility that they could test positive for THC if they are using CBD, said Dr. Michael Lynch, medical director of the Pittsburgh Poison Center at UPMC.

“People who are potentially going to be drug tested should be aware that even if they are using a predominantly CBD product it’s possible that it could trigger a positive urine drug screen,” Lynch said. “Employers and review officers should also be aware of this. There’s no way of determining the source of the THC once it’s in your body.”

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